In internet usage, particularly forum message boards, a user who swarms a post with useless commentary. Not considered a spam, as the user will usually post as part of a larger group of users, and the said user will usually think their comment is complimentary or in some way helpful, even though it is not.

The comment is:
1) Usually worthless. "OMG, that's great!" is not as complimentary as they poster believes it to be, and only serves to waste the time of the original poster.

2) Not well thought out. The comment is usually quick, and does not justify itself. No reason is given as to why the user feels the way he / she does about said topic, image, story, etc.

3) Part of a mob mentality. Piranhas don't work alone, unlike most spammers. Their posts are usually followed by a swarm of like-minded posts, fueling the useless flood of empty comments.

NOTE: These types of comments are usually most prevelant on a forum centered around skills development. (Art sites, writing boards, game modification boards, etc.) Where a person skilled in whatever the forum centers on is easily attacked by piranhas and his / her post is quickly devoured.

Original Poster: "Hey Guys, I just finished this 1200 DPI artwork, can you give some honest critique so I can improve myself?"

From the 'Apprentice'. One contestant observing the fish swimming at the London Aquarium, mused to a finalist'They're piranhas? But they look so friendly' You haven't seen their teeth' the finalist replies wearily. After 12 weeks in pursuit of the coveted job,he was covered in the nasty bite marks of his fellow contestants.

noun: Any wrestling or grappling technique (esp from Brazilian Jiu-Jitsu) that presses a body part or the ground against your mouth - sometimes yielding imprints of your teeth on the inside of your lips/gums or teeth marks on your opponents arms/legs.

1.s. f., Ictiol., Brasil,
teleósteo water fish cutting tooth candy, characterized for an extreme voracity and that it can constitute a real danger in the rivers and lakes where lives, therefore demonstrates great predilecção for animals that bleed;

A group of 3 bitches or more, unsatisfied with their sexual life, that go out to a restaurant and just wait for the waiter to make the slightest, simplest mistake to devour him alive. The members of the group consist of the bitchy piranha, the bitchiest piranha, and the nice piranha who is willing to let you live.
PS. any attempt to appeal the bitchy piranhas will result in a catastrophic event, so for your safety and the sake of your tips, try to appeal to the nice piranha only.

Damn! i have a group of piranhas on table 21 that is taking most of my time.